Unsaturated hydrocarbons such as ethylene and propylene are often employed as feedstocks in preparing value added chemicals and polymers. Unsaturated hydrocarbons may be produced by pyrolysis or steam cracking of hydrocarbons including hydrocarbons derived from coal, hydrocarbons derived from synthetic crude, naphthas, refinery gases, ethane, propane, butane, and the like. Unsaturated hydrocarbons may also be produced by the conversion of alcohols. Unsaturated hydrocarbons produced in these manners usually contain small proportions of highly unsaturated hydrocarbons such as acetylenes and diolefins that adversely affect production of subsequent chemicals and polymers. Thus, to form an unsaturated hydrocarbon product such as a polymer grade monoolefin, the amount of acetylenes and diolefins in the monoolefin stream is typically reduced. For example, in polymer grade ethylene, acetylene content typically is less than about 2 ppm, alternatively, less than about 1 ppm.
One technique commonly used to reduce the amount of acetylenes and diolefins in an unsaturated hydrocarbon stream primarily comprising monoolefins involves selectively hydrogenating the acetylenes and diolefins to monoolefins. This process is selective in that hydrogenation of the monoolefin and the highly unsaturated hydrocarbons to saturated hydrocarbons is minimized. For example, the hydrogenation of ethylene or acetylene to ethane is minimized.
The selective hydrogenation may be performed using catalysts comprising palladium (Pd) on an inorganic support. However, in some circumstances, such selective hydrogenation catalysts can produce green oil by oligomerizing the acetylenes and diolefins. As used herein, green oil refers to molecules having more than four carbon atoms per molecule. Green oil is one of a number of contributors to the fouling of the selective hydrogenation catalysts and the resulting catalyst deactivation. The deactivation of the selective hydrogenation catalyst results in the catalyst having a lower activity and selectivity to unsaturated hydrocarbons. Green oil production is particularly apparent in backend hydrogenation reactors. A selective hydrogenation catalyst in such backend reactors requires regular regeneration, and with each regeneration cycle, the selectivity and the life of the catalyst decreases. Therefore, a need exists for a selective hydrogenation catalyst that undergoes less deactivation in between regeneration cycles during the hydrogenation of acetylenes and diolefins.